There are several ways to indicate that you agree with what someone has just told you.
Genau is frequently used. It literally means Exactly, but is also used in conversation to mean Indeed or Of course.
In Ordnung, which literally means In order, is used with the meaning of OK.
Ich verstehe means I understand.
If you want to express disagreement you can say Stimmt nicht, That's wrong or Ich glaube nicht, I don't think so.